docs: def _add_to_transfer_queue(self, src_ase, dst_ase):
          # type: (SyncCopy, blobxfer.models.azure.StorageEntity,
          #        blobxfer.models.azure.StorageEntity) -> None
          """Add remote file to download queue
          :param SyncCopy self: this
          :param blobxfer.models.azure.StorageEntity src_ase: src ase
          :param blobxfer.models.azure.StorageEntity dst_ase: dst ase
          """
          # prepare remote file for download
          # if remote file is a block blob, need to retrieve block list
          if (src_ase.mode == dst_ase.mode ==
                  blobxfer.models.azure.StorageModes.Block):
              bl = blobxfer.operations.azure.blob.block.get_committed_block_list(
                  src_ase)
          else:
              bl = None
          # TODO future optimization for page blob synccopies: query
          # page ranges and omit cleared pages from being transferred
          sd = blobxfer.models.synccopy.Descriptor(
              src_ase, dst_ase, bl, self._spec.options, self._resume)
          # add download descriptor to queue
          self._transfer_queue.put(sd)
          if self._synccopy_start_time is None:
              with self._transfer_lock:
                  if self._synccopy_start_time is None:
                      self._synccopy_start_time = blobxfer.util.datetime_now()